1. Field of the Invention
This invention relates to a sheet material for use in a thermal transfer imaging system comprising a receiving sheet and a donor sheet. More particularly, it relates to a thermal imaging system wherein the donor sheet and receiving sheet do not stick to each other during thermal processing.
2. Description of the Related Art
Thermal transfer imaging processes wherein one or more thermally transferable dyes are transferred from a donor sheet to a receiving sheet in response to heat are well known. Such imaging processes employ imaging media consisting of a donor sheet comprising a dye or dyes and a binder for the dyes which is placed adjacent to a receiving sheet suitable for receiving the transferred dye(s). The imaging process comprises heating selected portions of the donor sheet in accordance with image information to effect an imagewise transfer of the dye(s) to the receiving sheet, thereby forming an image on the receiving sheet.
To enhance the image-receiving capability of the image-receiving sheet and thereby obtain higher density images, resins having a low glass transition point and softening point, e.g., polyester resins, are generally coated on the image-receiving sheet. However, when imaging is effected, heat is applied at high temperatures e.g., generally 200.degree. C. or higher when a thermal printhead is employed. The high temperatures cause softening and/or melting of the resin in the image-receiving sheet and the binder for the dyes in the dye donor sheet resulting in adhesion between the two sheets. This adhesion results in sticking and subsequent tearing of the two sheets upon separation from each other.
To eliminate this thermal sticking, it has been suggested to incorporate a dye-permeable release agent in either the donor or receiving sheet which allows for dye transfer but prevents adhesion of the donor sheet to the receiving sheet during printing. The release agent can be employed either as a discrete layer on top of the receiving material or the dye layer in the donor sheet, or the release agent can be blended in with the receiving material before coating.
Materials previously employed as release agents include silicone-based oils, poly(organosiloxanes), fluorine-based polymers, fluorine- or phosphate-containing surfactants, fatty acid surfactants and waxes. The inherently different chemical structure of the release agents from that of the dyes to be transferred leads to an interfacial barrier at the donor/receiver interface causing decreased dye densities in the image-receiving sheet. These materials are surface-active which promotes their presence at the receiving sheet/donor sheet interface where they additionally contribute desired slip properties and frictional characteristics to the image-receiving surface to prevent sticking. However, these release agents tend to be migratory and can be rubbed off the surface by touch, providing areas where sticking can occur. They also attract dirt and dust which degrade image quality.
Crosslinking of various release materials has been proposed to hold the release material in place and to alleviate some of the above problems. U.S. Pat. No. 4,626,256 issued Dec. 2, 1986, U.S. Pat. No. 4,820,687 issued Apr. 11, 1989, and U.S. Pat. No. 4,914,078 issued Apr. 3, 1990 disclose image-receiving layers containing dye-permeable releasing agents comprising hardened type (crosslinked) silicone oils. However, there are disadvantages to having a separate crosslinked material. Not only is there a decrease in dye density due to the inherently different chemical structure of the silicone oils from that of the dyes, but crosslinking additionally causes a decrease in the transferred dye density. The temperature requirements of thermally induced crosslinking processes limit the types of support materials that may be utilized for the receiving sheet. Moreover, certain release materials, most notably the silicone oils and crosslinked silicone oils, make it difficult to laminate the image-receiving sheet to other materials because they inhibit the laminating adhesive from adhering to the image-receiving sheet. Further, the release materials make it difficult to write on the image-receiving sheet because they interfere with ink adhesion at the image-receiving surface.
It has also been suggested to increase the heat resistance of the image-receiving material to prevent softening of the receiving material and hence alleviate sticking. U.S. Pat. No. 4,721,703, issued Jan. 26, 1988, discloses a receiving sheet comprising a base material and a coating composition, the coating composition consisting essentially of a thermoplastic resin for receiving a dye and a compound having two or more free radical polymerizable ethylenically unsaturated double bonds in one molecule, the coating being crosslinked. The resulting receiving sheet is described as being substantially non-heat bondable (does not stick) to the dye layer by virtue of the heat resistance imparted by the crosslinked polymer therein. However, this method is disadvantageous in that crosslinked materials generally result in decreased dye densities and require an additional processing step.
U.S. Pat. No. 4,997,807, issued Mar. 5, 1991, discloses a receiving sheet which is described as free from blocking (sticking of the receiving sheet to the donor sheet during thermal processing). The receiving sheet comprises a support having thereon an image-receiving layer formed by coating a substantially solvent-free coating composition comprising (A) a macromonomer dyeable with a sublimable dye and containing a radical polymerizable functional group at one terminal of the molecular chain thereof, said macromonomer being solid at room temperature, dissolved in (B) a liquid radiation-curable monomer and/or oligomer on a support and irradiating the coat with radiation. According to the examples given in the patent, excellent blocking results were obtained only when a polyfunctional monomer and a siloxane were present. This suggests that both crosslinking and a surface active agent (release agent) are necessary in order to obtain the best results.
U.S. Pat. No. 4,555,427, issued Nov. 26, 1985, discloses a heat transferable sheet (receiving sheet) comprising a receptive layer which receives a dye transferred from a heat transfer printing sheet upon being heated, the receptive layer comprising first and second regions having the following properties:
(a) The first region is formed from a synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C., preferably from -50.degree. to 10.degree. C., and having polar groups such as an ester linkage, C--CN linkage and C--C1 linkage.
(b) The second region is formed from a synthetic region having a glass transition temperature of at least 40.degree. C., preferably from 50.degree. to 150.degree. C., and preferably the second region-forming synthetic resin has also a polar group.
(c) Both the first region and the second region are exposed at the surface of the receptive layer, and the first region occupies at least 15%, preferably from 15 to 95% of the surface.
(d) The first region is present in the form of mutually independent islands, the respective longitudinal length of which is from 0.5 to 200 .mu.m, preferably from 10 to 100 .mu.m, and desirably the periphery of the first region is substantially surrounded by the second region.
According to the examples given in the patent, hardened silicone oils were added to enhance the releasability of the heat transfer printing sheet upon being heated.